Perhaps I am mistaken about the OS X compatibility. I only had a short and frustrating experience with the plotter long ago in OS 9.
You can always dedicate an old Mac as a plot server running OS 9 and PlotFlow. As I recall this is how we set it up way back when.
I would also strongly recommend that you try to find a way to get a new plotter/printer. The CadJet lacks the ability to print bitmaps and as I recall it also did not support the background empty fill (I might be wrong about this).
I've been using an Encad CadJet 2 for over 8 years and it's still working fine with ArchiCAD 8.1. My workstations are G4's using OS X.3. However, the CadJet 2 requires a serial port so I use an old beige G3 as the plot driver. All of my drawings are sent to a spool folder on the G3 over an ethernet network. This setup works great. The CadJet 2 is not all that fast, and it can not print photo images, but it is a very reliable printer.